In electrostatographic apparatus such as copiers and printers, automatic adjustment of process control parameters is used to produce images having well regulated darkness or optical density. Copier and printer process control strategies typically involve measuring the transmissive or reflective optical density of a toner image on an exposed and developed area (called a "test patch") of an image receiver. Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage, especially for transmission density, is that density is approximately proportional to the thickness of the marking material layer, over a substantial range.
Typically, toned process control test patches are formed on the photoconductor in interframe regions of the photoconductor, i.e., between image frame areas. An "on-board" densitometer measures the test patch density, either on the photoconductor or after transfer of the patches to another support member. From these measurements, the machine microprocessor can determine adjustments to the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
A transmission type of densitometer is particularly well suited to transmissive supports.
In this type, a light source projects light, visible or infrared, through an object onto a photodetector such as a photodiode. In a copier/printer, the photoconductor passes between the light source and the photodetector. When the photoconductor has toner on the surface, the amount of light reaching the photodetector is decreased, causing the output of the densitometer to change. Based on this output, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer such as described in U.S. Pat. No. 4,553,033 to Hubble, III et al, uses reflected light flux rather than transmitted light flux to determine density, and is particularly suited to opaque reflective supports.
One well-known approach to converting to a density measure uses an analog logarithmic amplifier, as suggested by the mathematical logarithm (log) function in the definition of optical density: EQU D=-log.sub.10 (T) Equation(1)
where D is optical density, and T is transmittance or reflectance (for transmission density or reflection density, respectively). The subscript "10" indicates that the logarithm is to the base 10. Since T must be between 0 and 1, the logarithm of Tis negative, and the minus sign (-) in equation (1) provides positive values for density, D.
The following U.S. Patents, for example, all teach the use of an analog logarithmic amplifier in a densitometer: U.S. Pat. Nos. 3,918,815 to Gadbois, 5,148,217 to Almeter et al, 5,173,750 to Laukaitis, and 5,903,800 to Stern et al. The high cost of precision analog logarithmic amplifiers does not seriously deter their use in expensive laboratory instruments. However, the high cost of analog logarithmic amplifiers has been an obstacle to the wide use of densitometers as built-in components within moderately priced copiers, printers, and other products.
Digital approaches to densitometer design have been advanced, as digital electronics improve in performance and decline in price, relative to analog logarithmic amplifiers. One digital approach in the prior art is to obtain a photodetector voltage signal representing intensity of transmitted or reflected light and convert this analog signal to digital form. The digital value is then used to enter a stored lookup table (LUT) of intensity and density values. The digital density value corresponding to the digital intensity value is read from the LUT. To cover a reasonably large range of density with the required resolution, an amplifier with selectable gain and a second LUT have been used.
U.S. Pat. No. 5,117,119 to Schubert et al discloses an automatic gain selection, i.e., an "auto-ranging" electronic circuit, along with a second LUT, to obtain high accuracy and resolution over an increased range of large densities. The first (or "base") LUT contains density values corresponding to an analog-to-digital (A/D) converter output for the lowest gain. The second (or "range") LUT is much smaller than the first LUT and contains the relative density corresponding to each available gain. It provides the additional density output associated with the gain selected. The two LUT outputs are summed to obtain the actual density measurement. U.S. patent application Ser. No. 09/185,842, now U.S. Pat. No. 6,222,176, (filed on Nov. 4, 1998 in the name of Rushing, et al) discloses an improvement wherein the density values are scaled in a manner that simplifies the addition the two LUT outputs.
The three ranges illustrated in the Schubert et al patent are divided by two threshold values in a 10:1 ratio. The circuit gains for the three ranges are in a ratio of 10:1, from one range to the next. Thus two ranges have 10:1 max-to-min light or voltage input ratios, and the third range (used for lowest light intensities or highest density) may have arbitrarily small input light or voltage level. The illustration in the Schubert et al patent shows a 10-bit A/D to attain resolution of 0.01 density units. The 10-bit analog-to-digital converter requires a "base" LUT of 2.sup.10 =1024 entries.
A major limiting factor in density resolution is the input voltage range that must be spanned by the A/D converter. Worst-case density resolution for each range comes at the high-density (low light intensity) end of the range, where the analog-to-digital converter resolution, i.e., one count, corresponds to the largest density increment. In the Schubert embodiment, where the A/D converter spans a 10:1 input voltage range, the density resolution at the low end of the A/D converter range (corresponding to high density) is 10 times as coarse as at the high end.
The Schubert et al patent uses multiple analog threshold levels for comparison to the light photodetector voltage signal. Low levels of electrical noise and circuit variability could degrade the comparator accuracy and reliability for the low-voltage thresholds.
With a gain ratio less than the 10:1 ratio of Schubert et al, resolution uniformity is improved. For example, a 2.0:1 gain ratio is suggested in U.S. patent application Ser. No. 09/185,926, now U.S. Pat. No. 6,225,618, (filed on Nov. 4, 1998 in the name of Rushing, et al). With the 2.0:1 ratio, density resolution at one end of the A/D converter range is 2.0 times as coarse as at the other end, and not 10 times as coarse as in Schubert et al.
With commonly available low-cost 1-of-8 analog switches, such as integrated circuit type MM74HC4051, available from Fairchild Semiconductor, a maximum of eight different gains are readily implemented using a single integrated circuit analog switch, with only a 3-bit gain select code. For eight gains in a 2.0:1 ratio, the corresponding range of transmittance or reflectance that can be measured is 2.sup.8 :1=256:1, corresponding to a density range of log.sub.10 (256)=2.4 density units However, many applications, such as within electrophotographic copier/printers, require a transmission density range of at least 3.0, corresponding to a transmittance ratio of 1000:1.
Since logarithmic conversion is at the heart of densitometry, logarithmic converters in other contexts may bear on densitometer applications. U.S. Pat. No. 5,341,089 to Heep discloses a digital circuit to convert an analog voltage input to decibel (dB) units. The dB output is defined as 10 times the logarithm (base 10) of the ratio of the power of the input signal relative to a reference power level This logarithmic conversion is of the same general type as used within densitometers, according to equation (1). The Heep disclosure has no selectable gain and no auto-ranging. Large inputs must first be scaled down by a manually adjusted voltage divider to obtain an input within the operating range, and an output from a second LUT is added to compensate for the scaling down. Interpolation between LUT values is applied to obtain the desired accuracy, adding complexity to the circuit and lengthening the time required obtaining a measurement update.